As is well known in the art, the operation of inlet and exhaust valves in internal combustion engines is often controlled by a rocker arm that reciprocates about a rocker shaft. A first end of the rocker arm, located on a first side of the rocker shaft, is reciprocated by a push rod connected to a cam follower, which in turn is driven by a cam mounted on a camshaft. The second end of the rocker arm, located on the second side of the rocker shaft, drives the valve stem of an inlet or exhaust valve that is spring-biased into a normally closed position. Each inlet valve and each exhaust valve has an associated rocker arm. When the valves associated with a particular piston are fully closed (i.e. when the piston is in its top dead center (TDC) position on the compression stroke of a four stroke engine), a certain predetermined clearance is required between the second end of the rocker arm and the end of the valve stem which is contacted by the rocker arm in operation of the engine. This clearance must be set within fine tolerances, typically of the order of +/−2/1000 inch (0.051 mm). The process of setting this clearance is referred to herein as “valve clearance setting” and is commonly referred to in the art as “tappet setting” in the United Kingdom or “valve lash setting” in the USA.
The valve clearance is typically adjusted by means of a threaded adjustment screw that extends through the first end of the rocker arm and is seated in a cup formed in the end of the push rod. The adjustment screw may be locked in the required position by a lock nut, or may be a friction screw or the like which does not require a lock nut.
The combination of the cam, cam follower, push rod, adjustment screw, rocker arm and rocker shaft is referred to herein as the “valve drive train”.
Conventionally, valve clearances are adjusted manually, by use of a feeler gauge which is inserted between the second end of the rocker arm and the end of the valve stem whilst manually adjusting the adjustment screw at the first end of the rocker arm. This process is labor intensive, time consuming and relatively inaccurate/inconsistent. It would clearly be desirable to automate the process of valve clearance setting. To date, however, attempts at automation have failed to deliver satisfactory results.
One previously proposed method of performing automatic valve clearance setting utilizes an automatic machine tool for adjusting the adjustment screw, a linear position sensor which senses the position of the second end of the rocker arm and a linear actuator having a clip member which engages the rocker arm on the second side of the rocker shaft and which is capable of pushing the rocker arm in its valve-actuating direction and pulling the rocker arm in the opposite direction. This method comprises the steps of pushing the second end of the rocker arm in its valve-actuating direction to a predetermined zero position (reference datum) in which the second end of the rocker arm contacts the end of the valve stem but does not displace it from its normally closed position, pulling the rocker arm in the opposite direction by an amount sufficient to remove all backlash from the valve drive train, and adjusting the adjustment screw against the pulling force until the position sensor indicates that the second end of the rocker arm is at a predetermined distance (the required valve clearance) from the zero position. As used herein, “backlash” refers generally to clearances between adjacent, mutually coupled components and is not restricted to clearances between relatively rotatable components. The backlash in the valve drive train additionally includes backlash between the rocker shaft and its mounting pedestals.
This previous method has been found to be unsatisfactory in practice, failing to provide consistently accurate setting of valve clearances. The present inventors have determined that this prior method does not take sufficient account of variations in the relative positions of the various elements of the valve drive train caused by backlash in the valve drive train and movement of the rocker arm during the setting process, and does not take sufficient account of variations in the dimensions of the valve drive train elements between individual valves of an engine and between different engines.